1. Field of the Invention
This invention relates to a continuous process and apparatus for the bleaching of triglyceride oils with a bleaching adsorbent.
2. Description of the Prior Art
In the processing of oils and fats for purposes of producing salad and cooking oils, and other edible oil products such as margarines and shortenings, it is necessary to bleach the oil with an adsorbent substance such as an adsorbent clay. The purpose of the bleaching process is to remove coloured materials from the oil, such as chlorophyll and chlorophyll breakdown products, brown-coloured compounds and other materials which may be sufficiently polar for removal from the oil by adsorption on the clay. If these compounds are not removed, the desired light colours cannot be achieved in the various oil and fat products mentioned above. Equally importantly, the chemical activity of these compounds can lead to the formation of additional coloured material, and to the formation of compounds which impart unacceptable flavours to the oil products. They must therefore be removed to very low levels.
It is generally not intended to remove carotene, which is a major contributor to the colour of most oils in the crude state, by this process. Carotene does not readily adsorb on bleaching clay, but it breaks down on heating to near-colourless (yellow) compounds. Hence, the high temperatures to which oils are exposed in the course of deodorizing after bleaching are relied upon to eliminate the strong carotene (red) colour of many oils.
The complexity and variety of the compounds to be removed by bleaching are such that it is not practical to analyze the bleached oil for them in detail to determine the effect of the process. Instead, it is customary to determine the colour of the oil after bleaching by comparison with Lovibond-Red standards as described in Bailey's Industrial Oil and Fat Products, Third Edit., (1964), pages 132 and 133. Because of the difficulties with colour comparisons in the presence of carotene, it is often desirable to determine the amount of chlorophyll left in the oil by analysis and to determine the peroxide value (PV) as a measure of primary oxidation products, and the anisidine value (AV) as a measure of secondary oxidation products in the bleached oil. If bleaching is performed inefficiently, or without proper protection from oxygen, it can be expected that coloured materials of a non-carotenoid nature such as chlorophyll are inadequately removed and that oxidation values will be relatively higher in the bleached oil.
Because of this complexity in evaluating bleached oils, the ultimate test must be an evaluation of the oil after deodorizing. At that stage, it is possible to assess the effect of the bleaching process properly with respect to colour since there is no longer any interference from carotenoid compounds. Also, the effect on flavour and flavour stability can be evaluated at this stage.
In a typical process of bleaching as carried out in the industry, adsorbent clay is mixed with the oil or liquified fat (hereinafter designated generically as oil) which usually has been subjected to a refining operation, the mixture is adjusted to the desired bleaching temperature and is held at this temperature for a sufficient length of time for adsorption of coloured material to take place to the maximum extent. At the end of this period the oil is filtered to remove the clay.
It is usually preferable to protect the oil from contact with air throughout the process, and particularly during that phase of the process when the oil is at maximum temperature and in contact with the bleaching clay. Usually, this is accomplished by processing under vacuum either in batch kettles or continuously in stirred, flow-through tanks which may be compartmented to achieve a degree of control over the residence-time of the oil/clay mixture. The use of vacuum also performs the important function of removing any air from the oil/clay mixture and of removing moisture. It is, however, important to avoid complete drying of the oil/clay mixture, since this reduces the adsorptive capacity of the clay according to many investigators (see, for example, Bailey's Industrial Oil and Fat Products, Third Edit., p. 780).
A bleaching process in which the bleaching action takes place under atmospheric or greater pressure is described in Harris et al, U.S. Pat. No. 3,673,228. In this process, there is a preliminary vacuum treatment which serves only to deaerate and to moisture-adjust the oil/clay mixture for optimum bleaching efficiency. This can be achieved by putting the oil/clay mixture through a vacuum-dryer rather than having the entire bleaching section of the process under vacuum. With this arrangement, more precise control of the moisture-adjusting phase of the process is possible, since there is no need to have the oil/clay mixture under vacuum during the entire bleaching phase of the process in order to maintain protection from air.
With conventional bleaching processes there are two serious difficulties. First, it is difficult to meter the bleaching clay, which is a fine powder, from ambient pressure into the evacuated vessel in which the oil/clay mixture is to be dried, or, in which both drying and bleaching are to take place. Usually this can be done by installing devices which measure-out a small quantity of clay and transfer it from ambient pressure into the evacuated vessel. This means that the bleaching operation must be semi-continuous, which is more complex mechanically. Another method, as suggested in Harris et al, U.S. Pat. No. 3,673,228, is to slurry the clay in a small quantity of the oil in a separate tank, and then meter this slurry continuously into the main oilstream. The disadvantage of this method is that changes in the oil-stocks to be bleached require also a change in slurry-stock, if contamination between oil-stocks is to be avoided, which complicates the process considerably. Also contact of a portion of the oil with the clay is far in excess of the optimum time and amount of clay, for bleaching.
The second difficulty arises with respect to the contact-time of the oil with the clay during bleaching. The selected residence-time of the oil-clay mixture in the bleaching zone in the usual conventional processes employed in the industry is in the range of about 5 to 30 minutes, depending on the type of oil and type of clay. Agitated tanks allow considerable short-circuiting and back-mixing with the result that the actual residence-time of the increments of the oil/clay mixture varies widely. The use of a packed column, as described in Harris et al U.S. Pat. No. 3,673,228, provides some improvement, but there is still considerable variation in residence time between different portions of the mixture. The result of this variation is that bleaching vessels are sized for rather long average-residence times. This makes it inevitable that portions of the oil are exposed to bleaching conditions for so long that certain reactions which produce coloured material in the course of bleaching can assume significant proportions. Consequently the bleaching process is then correspondingly less efficient. Also, packed columns do not allow for efficient oil/clay mixing and hence longer bleaching times must be allowed to achieve proper clay-utilization. The object of the present process is to overcome these disadvantages.